Tory ministers want to fiddle the figures on child poverty because they have no hope of keeping a promise to end it, a top economist said yesterday.

Work Secretary Iain Duncan Smith says rules, which class a child in poverty if the family lives on less than 60% of average income, are too simple.

He wants other factors, such as parents splitting up, included.

But Paul Johnson, head of the respected Institute for Fiscal Studies, said it was clear that the looming child poverty target is also behind the decision to change the way it is measured.

Ministers are supposedly committed to ensuring that no child grows up in poverty by 2020 but their policies will push 400,000 below the breadline, the IFS think-tank has predicted.

Mr Johnson said: “This Government, alongside the previous government, signed up to a target just before the last election – a target which most of them knew that they couldn’t meet, let’s be honest – which was to dramatically reduce child poverty on this current measure by 2020.

“Now, they know that they can’t do that because they can’t spend vast amounts of money on welfare to achieve it. So that’s clearly one of the things behind this.”

At the moment children are classed as living in poverty if their family live on less than 60% of average income.

Former Tory leader Mr Duncan Smith will keep the measure but today claimed that “it doesn’t actually measure what’s going on in the family”.

“There are many factors that impact on a child’s wellbeing and ability to succeed in life... and measuring income alone does little to represent the experience of those in poverty,” he said.

Mr Johnson said that was not a bad thing but risked blurring the picture.

“What it would, I think do, would not give us a single poverty line – we wouldn’t be able to say ‘there are exactly this number of people in poverty because we don’t have a single measure’.”

TUC general secretary Brendan Barber accused the Government of "moving the goalposts" on child poverty.

"It is ludicrous to say that child poverty isn't about how much money families have. Children who grow up in low-income households are far more likely to have to go without," he said.

"Instead of moving the goalposts in a bid to excuse its own failings, the Government should be concentrating on addressing the causes behind child poverty, like high unemployment and soaring living costs."

Labour’s Stephen Timms said he didn’t object to the Government looking at other measures but warned welfare cuts will hit youngsters.

“What they mustn’t do is try and obfuscate what is actually happening about poverty at the moment.

"The Institute for Fiscal Studies says that child poverty is going to rise by 400,000 over the course of this Parliament.”